There already exist many known video data compression techniques. These include numerous video encoding techniques that use a blockwise representation of the video sequence such as for example techniques implementing video compression standards laid down by the MPEG organization (MPEG-1, MPEG-2, MPEG-4 part 2, etc) or the ITU-T (H.261 . . . H.264/AVC) standards. Thus, in the H.264 technique, each image can be divided into slices which are themselves divided into macroblocks which are then sub-divided into blocks. A block is constituted by a set of pixels. According to the H.264 standard, a macroblock is a square block with a size equal to 16×16 pixels which can be divided again into blocks sized 8×8, 16×8 or 8×16, the 8×8 blocks being then capable of being re-divided into blocks sized 4×4, 8×4 or 4×8.
According to the prior art techniques, the macroblocks or the blocks can be encoded by intra-image or inter-image prediction. In other words, a macroblock or block can be encoded by:                temporal prediction, i.e. with reference to a reference block or macroblock belonging to one or more other images; and/or        a prediction known as a “spatial” prediction as a function of blocks or macroblocks neighboring the current image.        
In the latter case, the prediction can be done only on the basis of blocks which have been previously encoded.
More specifically, according to the H.264 technique, images I are encoded by spatial prediction (intra prediction) and images P and B are encoded by temporal prediction relatively to other images I, P or B encoded/decoded by means of motion compensation.
In order to be encoded, these images are sub-divided into blocks. For each block there is encoded a residual block also called a prediction residue corresponding to the original block minus a prediction. The coefficients of this block are quantified after a possible transformation and then encoded by an entropic encoder.
According to the H.264 technique for example, the following are encoded for each block:                the type of encoding (intra prediction, inter prediction, prediction skip);        the type of partitioning;        information on prediction (orientation, reference image, etc);        motion information if necessary;        the encoded coefficients;        etc.        
The decoding is done image by image, and for each image it is done macroblock by macroblock. For each macroblock, the corresponding elements of the stream are read, and the inverse quantification and the inverse transform of the coefficients of the blocks of the macroblock are done. Then, the prediction of the macroblock is computed and the macroblock is rebuilt by adding the prediction to the decoded prediction residue.
With the coming of new high-resolution video formats, the size of the blocks used in current-day video compression standards is unfortunately no longer suited to the content of the video sequences to be compressed.
Furthermore, the various types of prediction possible in the AVC encoder are not the same depending on the size of the blocks. For example, in intra encoding, it is not possible to benefit from as many directions of prediction for a block sized 16×16 as for a block sized 4×4. Now, the use of 4×4 sized blocks to encode a high-resolution sequence cannot be used to efficiently exploit the signal to be compressed since the quantity of information contained in the block is not sufficiently representative of the high-resolution sequence for the encoding applied to be efficient in terms of bit-rate/distortion costs.
Furthermore, the maximum size of the blocks for the prediction is limited to 16×16 in current-day video compression standards. Now the use of bigger-sized blocks may have some utility in the case of high-resolution sequences.
The document by A. Tourapis, J. Boyce, “Reduced Resolution Update Mode Extension to the H.264 Standard” proposes to extend the encoding technique known as “Reduced Resolution Update” or RRU to H.264 encoding.
To encode a block, the RRU technique makes the prediction and computation of the prediction residues of the block at the resolution of the block. Then the block is sub-sampled. The sub-sampled coefficients are transformed, quantified and encoded by an entropic encoder. At the time of decoding, the coefficients are decoded, the inverse quantification and transformation are performed and then a step of oversampling the coefficients is applied in order to rebuild the block at its initial resolution.
The extension of this RRU technique to an H.264 encoder consists of the application of the RRU technique to M×N sized blocks with M and N being multiples of 16.
Thus, in this technique, all the blocks of the image have the same size M×N which is not necessarily well suited to the content of the image.